var fs=require('fs')

function pReadFile(filePath) {
     return new Promise(function (resolve, reject) {
        fs.readFile(filePath,'utf8',function (err,data) {
            if (err){
                reject(err)
            } else {
                resolve(data)
            }
        })
    })
}

pReadFile('./data/a.txt')
    .then(function (data) {
        console.log(data)
        return pReadFile('./data/b.txt')
    })
    .then(function (data) {
        console.log(data)
        return pReadFile('./data/c.txt')
    })
    .then(function (data) {
        console.log(data)
    })
